If a customer paid online, you will need to refund them from the Orders2me back-end. Their credit card information doesn't pass to your POS system, so you will not be able to make any adjustments or refunds on the transaction.
How to submit a refund via Orders2me back-end https://support.orders2.me/hc/en-us/articles/209916483-How-Do-I-Submit-A-Refund-For-An-Online-Order-